1. Technical Field
The present invention relates to a network of private branch exchange systems using ISDN lines, and more particularly, relates to a method for networking a plurality of the private branch exchange systems for sharing common resources by connecting mutually the private branch exchange systems using ISDN lines.
2. Related Art
Generally, a conventional telephone network is a dedicated service network intended to provide only specific communication service to terminal subscribers. Typically, private branch exchange systems are connected to a public switched telephone network (PSTN) via private wires to provide voice communication services. In contrast to a conventional telephone network, integrated services digital network (ISDN) offers a variety of new features including multimedia communication service such as voice, high speed data and image communication services and other additional non-voice communication services via network interfaces on the basis of digitization of the telephone network. These new features have attracted contemporary private branch exchange systems for integration to benefit from ISDN services such as disclosed, for example, in U.S. Pat. No. 5,305,313 for Electronic Switching System For Use In Connection To An ISDN And Method Of Setting Communication Disconnection Reasons issued to Katoh, U.S. Pat. No. 5,450,396 for Communication System And A Private Branch Exchange To Be Used In Such A Communication System issued to Havermsan, U.S. Pat. No. 5,481,605 for Private Branch Exchange Capable Of Analyzing Information Received From ISDN issued to Sakurai et al., and U.S. Pat. No. 5,621,731 for Private Exchange For ISDN issued to Dale et al.
As the ISDN services become widely in use, the private exchange and its terminal stations connected to the ISDN must effectively accommodate multimedia communication services and a variety of other additional services through the ISDN. ISDN interfaces must be provided in the private branch exchange system on the basis of network interface standards. Exemplary ISDN interfaces for private branch exchange system are disclosed in U.S. Pat. No. 5,305,312 for Apparatus For Interfacing Analog Telephones And Digital Data Terminals To An ISDN Line issued to Fornek et al., U.S. Pat. No. 5,450,486 for Telecommunication System And A Linecard issued to Maas et al., U.S. Pat. No. 5,319,640 for Transmitting/Receiving System Having Digital Switching Network issued to Yamasaki et al., and U.S. Pat. No. 5,550,822 for Private Branch Exchange issued to Ikeda. ISDN interface facilities may be used to facilitate communications and signaling between a plurality of private branch exchange systems in a network as disclosed, for example, in U.S. Pat. No. 5,422,943 for Private Branch Exchange Networks issued to Cooney et al., and U.S. Pat. No. 5,455,855 for System For Connecting Public Network Subscriber And Private Network Subscriber issued to Hokari. While these ISDN interfaces are implemented in accordance with network interface standards and networking between private branch exchange systems are possible, I have noted that further improvement in a method for enabling networking between a plurality of private branch exchange systems each using an ISDN line as a private wire can still be contemplated.